Our ultimate goal is the complete understanding of neural mechanisms for both species-specific (bisonar) and common (communication) auditory functions. Unlike the auditory periphery, the central auditory system contains many different types of neurons. Some neurons are tuned to particular acoustic parameters characterizing sounds other than frequency or to specific parameters characterizing combinations of signal elements in complex sounds. Different types of neurons are clustered in different cortical areas and form maps for systematic representations of acoustic parameters. It has been explained that all these neurons and maps resulted from the neural interactions within the ascending auditory system. However, we had found that, according to auditory experience, the descending (corticofugal) system changes response properties of subcortical neurons and subcortical maps, accordingly those of cortical neurons and cortical maps. Our aim of the proposed research is to explore the functions of the corticofugal system and to test our hypotheses (1) that the auditory cortex, through the corticofugal system, adjusts and improves auditory signal processing in the frequency, amplitude and time domain according to auditory experience, (2) that this mechanism is altered by the modulatory systems (cholinergic, serotonergic, dopaminergic and noradrenergic systems) if the acoustic become behaviorally relevant to the animal, and (3) that in addition to corticofugal modulation, colliculofugal modulation, improves and adjusts subcolliclar auditory signal processing. Our proposed research will contribute further toward our understanding of the neural mechanisms for processing behaviorally relevant sounds and the functional organization of the central auditory system. We will be able to propose possible neural mechanisms for the reorganization of the auditory system caused by hearing disorders. We will study the response properties of cortical and subcortical auditory neurons, and then study how their response properties are changed by focal electric stimulation of the auditory cortex, conditioning, sensitization and focal applications of different types of drugs to cortical and subcortical auditory neurons. All the data obtained will be evaluated in relation to our hypotheses.